This invention relates to a rendering process and more particularly to a rendering process for treating animal and/or vegetable material.
The aforementioned materials generally contain two or all of the following phases: fat, fat-free solids, and water. In the case of animal by-products, for example material from slaughter houses, packing houses and the like such as a mixture of beef and mutton offal, the phases after rendering are a liquid phase which consists essentially of fats and oils and a solid phase which consists essentially of meat and bone meal. The solid phase is usually high in protein and processed into animal feed. The liquid phase is separated into tallow and waste process water.
Rendering of animal tissues by heat is carried out presently by two types of processes: dry rendering high temperature (135.degree. C.) and low temperature (70.degree.-100.degree. C.) wet rendering.
In dry rendering, the raw material is heated, separated into the tallow, fat-free solid and water phases and dried in the same operation. This process suffers from several drawbacks: 10-15% residual fat in meal; raw material must be washed thoroughly to ensure that the tallow products is of premium grade; the nutritive value of the meal is downgraded by the high temperature of the process; the process required high grade steam and has a higher energy consumption compared with low temperature rendering systems.
The low temperature rendering systems include: the traditional "digester" type direct steam injection process which produces high losses of product in the stick-water; the Pfaudler process (U.S. Pat. No. 3,519,662) and the Alfa-Laval "Centritherm" and "Centribone" process (N. Z. Patent Specification No. 186249). The latter processes are generally regarded as "wet rendering" as water is introduced into the raw material and live steam is directly injected into the raw material.
The use of steam requires that the processing plant has a facility to raise process steam. In addition the injection of live steam or extra water into the flow of raw material increases the amount of process water and can lead to a high B.O.D. level in the process water so that further separation must take place before the process water can be released into existing sewers and effluent treatment plants.
The main object of the present invention is to provide a low temperature rendering process in which no process water and no live steam is introduced into the raw material being rendered.
In one broad aspect the invention provides a low temperature rendering process for separating animal or vegetable raw material into a fat fraction and at least one protein fraction comprising reducing the particle size of the raw material, feeding the reduced raw material into a rendering vessel, feeding under pressure into the rendering vessel recycled tallow or recycled tallow phase, heating the resultant raw material and tallow mixture by the rendering vessel using heat exchange means, said mixture being heated at a temperature which is in the range of 75.degree. to 100.degree. C., the mixture being subjected to particulate fluidization by the recycled tallow and continuous agitation thereof within the rendering vessel, said heating being carried out without the introduction into the rendering vessel of any process water and live steam and moving a flow of the heated material from the rendering vessel to a separator for phase separation.
Acid can be added to the raw material prior to being comminuted in size. The raw material is preferably acidified to a pH of 3.8 to 4.5.